foo.cc: In function 'int read()':
foo.cc:8:19: warning: 'c' is used uninitialized [-Wuninitialized]
    8 |     while(c < '0' or c > '9') {
      |           ~~~~~~~~^~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:8:19: warning: 'c' may be used uninitialized [-Wmaybe-uninitialized]
    8 |     while(c < '0' or c > '9') {
      |           ~~~~~~~~^~~~~~~~~~
foo.cc:6:19: note: 'c' was declared here
    6 |     register char c;
      |                   ^

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